A study was begun to determine what interactions of virus components are required to initiate budding at the cell surface. A temperature-sensitive mutant of Rauscher MuLV, ts 25, has been shown to develop, even at nonpermissive temperatures, upon incubation with specific and nonspecific ligands. Furthermore, budding can proceed at 0 degrees C, suggesting that organization of pre-formed virus components is by nucleation, and not energy-dependent. A similar study of virus component interactions was initiated with a cell line of hamster origin (HTG2) which produces large numbers of noninfectious virus. In this instance virus is apparently produced (a) with little or no gp70, and (b) without cleavage of core gag precursor. As a result of the latter, native core structure is abnormally retained. A study has also begun to determine the bases for noninfectivity of virus produced in several cell lines of porcine and primate origin.